I used to believe, having only one TV in the house, that televisions worldwide could only receive one signal at once, and that changing the channel would change it for all of them. This was my explanation for commercial breaks and the like.
A childhood belief about tv

others believed things like this
- Different televisions contain different TV shows.— IMMarkotron
- I used to think being on holiday meant that you were moving house temporarily, like with a change of address.Anon
- that actors went to the toilet during the ad breaks.— ash
- I used to think that if a mirror with me in it breaks, I break.Anon
- Since we receive bread at Mass, I thought he was called "Jesus Crust" Seemed pretty obvious!Anon
- I used to think that the turn signal told my mom where to go.— Jenna